In line pressure regulator shut-off valves (PRSOV) are flow control devices with an internal closed control loop to allow control of the output pressure (p_out), in principle regardless of the upstream inlet pressure (p_in). Output pressure control is generally performed by a regulation piston which senses two different pressures on its faces: the outlet downstream pressure (p_out) and the regulation chamber pressure (p_reg). Any imbalance of these forces causes piston movement which in turn alters the flow from inlet to output and therefore redresses the imbalance in forces. In other words the piston position is self-adjusted in order to have p_out=p_reg.
The regulation pressure p_reg is typically maintained by a pressure relief valve that vents any overpressure.
The stability, i.e. the absence of oscillations of such regulation devices is highly dependent on the regulation chamber volume: the higher the volume, the more unstable the valve. Instability of the valve results in high frequency oscillations of the valve piston, which in turn leads to accelerated wearing of both the seals and the piston bore.
Such valves typically have a shut off function which is effected by venting the regulation chamber to low pressure. This may be achieved by opening a solenoid valve on a pneumatic line in communication with the regulation chamber. When the PRSOV shut off function is performed by a remotely installed solenoid, pneumatically linked to the control chamber by means of a sense-line, the stability margin can be insufficient, resulting in an unacceptable risk of unstable valve operation.